(1) Field of the Invention
This invention relates to a thread supply device used in a sewing apparatus having a plurality of needles and at least one looper, in which needle threads guided to the needles and a looper thread guided to the looper are cut by a thread cutting unit at a cutting position, which is off a thread supply path, after a sewing operation is finished but before the following sewing operation starts.
(2) Description of the Prior Art
In a known sewing apparatus with a plurality of needles and a looper, needle threads and a looper thread are each inserted between two discs of a thread tension adjusting unit, whereby lengths of the threads to supply are adjusted. After a sewing operation, the threads are cut by a cutting unit.
For cutting a thread as above, the thread should be pulled to a cutting position which is off a thread supply path, which requires an extra length of the thread. In order to supply the extra length, Japanese Patent Publication Kokai No. 51-30047 discloses a sewing apparatus equipped with a thread feeding unit having a thread hook for each thread. As shown in FIG. 1, such thread feeding unit comprises a thread hook 101 and two thread guides 102 and 103. The thread hook 101 is pulled down in a direction of the arrow in a desired distance, whereby forcibly saving the thread 100 in a length required for a cutting operation. Then, the thread 100 is moved in a direction which is substantially perpendicular to a thread supply path and cut by a cutting unit.
The thread feeding unit of FIG. 1 works if the thread 100 is not loose before being pulled by the thread hook 101. If the thread is loose as shown with a dashed line in FIG. 1, pulling the hook 101 only makes the thread 100 taut and the thread 100 is cut in an insufficient length. As a result, the thread comes off from a thread hole of the needle or the insufficient length of thread contracts a final portion of the sewing medium so as to give the sewing product a poor appearance.
When the next sewing operation starts, the looper starts working in association with a main shaft of the sewing apparatus. If a looper thread is insufficient in length, the looper thread comes off from the looper, which unables the sewing operation. To solve this problem, the looper thread is supplied extra for the cutting operation. Such a process leads to another problem that the looper thread gets loose in the middle after being cut and caught by the thread guide or the like to be cut erroneously.